1. Field of the Invention
This invention relates to a silver halide photographic emulsion and particularly to a direct positive silver halide photographic emulsion to which fog was previously applied.
2. Description of the Prior Art
When a silver halide light-sensitive element is exposed to light including that within the region of the light-sensitive wavelength of the light-sensitive element and developed, the photographic density thereof increases in accordance with the increase of the exposure quantity, and finally reaches its maximum value. In this case, however, when the exposure quantity further increases, the photographic density decreases again and finally a positive image is formed. Such a phenomenon is generally called "solarization". Furthermore, a similar reversal phenomenon as results from the exposure to light is observed in a silver halide emulsion optically or chemically fogged during the production process.
It is well-known that when a sensitizing dye having a sensitizing function with respect to an ordinary negative-positive type emulsion, such as a carbocyanine dye, is added to a previously fogged photographic emulsion an increase of reversal sensitivity and magnification of the region of reversal light-sensitive wavelengths are observed.
According to U.S. Pat. No. 2,323,187 a carbocyanine dye is advantageous for reversal sensitization in a larger quantity range than in an ordinary negative-positive type emulsion. In general, a sensitizing dye, particularly in the case where the quantity added is large, leaves residual coloring in a colloidal image supporting layer and/or auxiliary layer. Such a residual color effect is unfavorable in, for example, photographic papers. That is, a bright white color cannot be obtained in monochromatic photographs and, on the other hand, the reproduction of true color is impossible in color photographs.
When pink or yellow color remains in a photographic paper, another disadvantage is observed, that is, in the case a part of a completed photograph is irradiated with sunlight or the like, the dye of the corresponding part is bleached by the light, and, as a result, an image, one part of which is white and the other part of which is colored, is obtained. Accordingly, in the case when a photographic emulsion which was previously fogged with a sensitizing dye is sensitized, it is important that the occurrence of coloring in the finished photograph be avoided.
Furthermore, as shown in Japanese Pat. No. 4125/1968, a reversal sensitization is attained more advantageously in the case when an emulsion containing a rhodium compound is sensitized with a cyanine dye than in the case where a desensitizing organic compound having a desensitizing function when added to an ordinary negative-positive type emulsion, for example, pinakryptol yellow, and a cyanine dye are jointly employed.
In the case when the emulsion containing a rhodium compound is sensitized with a cyanine dye, it is important, from a technical viewpoint, that a desensitizing organic compound such as pinakryptol yellow be further added to the emulsion in order to lower the minimum concentration of its reversal curve, to thereby obtain a clearer image. In this case, there is a tendency for the desensitizing organic compound to deteriorate the reversal sensitivity caused by the cyanine dye. Therefore, in the above-mentioned case, a sensitizing dye, the deterioration of the reversal sensitivity of which is small, even when a desensitizing organic compound is also present, is required.